Digital halftoning is the process of converting a monochrome, continuous-tone, raster image or photograph into a binary pattern of printed and not printed dots. In the case of color printing, images may be produced by superimposing multiple halftone images where each component halftone corresponds to a specific color of ink such as, for example, cyan, magenta, yellow, and black (CMYK).
When categorizing the various methods of halftoning, methods that produce varying shades of ink intensity using a periodic pattern of printed-dot clusters that vary in size according to tone are generally referred to as amplitude modulated or AM halftoning. The process of converting the continuous-tone picture elements (pixels) of the original raster image into printed and not printed dots may typically be performed using a pixel-wise thresholding operation comparing a subject pixel with a threshold element from a dither matrix whose row and column index within the matrix corresponds to the row and column coordinate of the subject pixel using some predetermined mapping operation.
In the case of frequency modulated or FM halftoning, printed dots may be arranged in a dispersed, often aperiodic, pattern, where shades of gray may be represented by same-sized printed-dot clusters whose spacing apart varies with the ink intensity level. In addition to being performed by a dither matrix, FM halftoning may also be performed using error-diffusion, where a subject pixel may be quantized to a binary printed/not printed value with the resulting quantization error added into soon-to-be-processed pixels within a local neighborhood about the subject pixel.
With the advent of modern digital printers based upon electrophotographic (laser) as well as inkjet printing mechanisms, the process of converting a continuous-tone image into printed dots is complicated by these modern primers' ability to print dots of variable size. In the case of laser printers, it is now possible to modulate the intensity of the laser beam and, thereby, modulate the size of the printed spot formed on the photosensitive drum. With regard to inkjet printers, it is possible to now modulate the volume of ink dispersed in each ejected droplet or to deposit multiple droplets at the same location on the page.
By depositing a larger volume of ink at a given location, an inkjet printer can modulate the size of the resulting printed dot. Inkjet printers are also readily available that, in addition to modulating the size of ejected droplets, make use of two or more separate inks representing the same primary color but with different color intensities. For example, six color inkjet printers typically employ two shades of cyan (cyan and light-cyan) and two shades of magenta (magenta and light-magenta). Eight color inkjets typically employ three shades of black (black, light-black, and light-light-black).
With regard to the conversion of a monochrome, continuous-tone, raster image or photograph into patterns of printed and not printed, variably-sized, variable color intensity dots, first-generation processing means may typically employ traditional methods of halftoning incorporating multi-level quantizers. Such methods are sometimes referred to as the process of digital multi-toning. The problem with these first generation methods is that they may often produce banding artifacts in areas of smooth gradients when transitioning from one available dot size or color-intensity to another. For this reason, some later generations of multitoning processes created patterns composed of at least three different available dot sizes/color-intensity dots, but this novelty has typically occured at the price of increased computational complexity for the quantizing means.